In vitro translation studies on the RNA prepared from the Harvey strain of murine sarcoma virus have yielded a protein coded for by this defective transforming virus for the first time. The protein has been translated from the sarcoma specific information of the Harvey virus and therefore represents a marker for the first time of potentially transforming information of an RNA tumor virus. This will now enable tumor virologists to purify a protein coded for by a transforming RNA tumor virus which is actually responsible for the change of the normal cell to a malignant cell. The purification and development of assays for such proteins opens new approaches to the identification of the viral cause of neoplasia. BIBLIOGRAPHIC REFERENCES: Young, H. A., Shih, T. Y., Scolnick, E. M. and Parks, W. P.; Steroid induction of mouse mammary tumor virus: Effect upon synthesis and degradation of viral RNA. J. Virol. 21: 139-146, 1977. Troxler, D. H., Parks, W. P., Vass, W. C. and Scolnick, E. M.: Isolation of a fibroblast nonproducer cell line containing the Friend strain of the spleen focus-forming virus. Virology 76: 602-615, 1977.